Drilling fluids, or drilling muds, are used in the drilling operations of oil and gas wells in order to lubricate and to cool the drill bit and its fittings, during rock penetration and, at the same time, to keep rock cuttings in suspension while at the same time carrying same out of the borehole.
In addition to those basic functions, drilling fluids must be able to be in charge of several other requested jobs during the drilling procedure and, to have it successfully done, some desirable physical and chemical properties are required, as, for example:
a) the variation of the drilling fluid viscosity with temperature should be small enough to allow such fluids, when submitted to a considerable temperature variation, like those that usually happen in the off shore deep water drilling, to be quickly recirculated throughout the whole system, with no overcharging for the pumping system;
b) inside the well, the gel strength of the drilling fluid should be sufficient to hold in suspension the rock cuttings produced during drilling, if, eventually, the circulation of the drilling system is stopped;
c) the density of the drilling fluid should be high enough to exert suitable pressure on the borehole sides to prevent the entrance of fluids from the earth formation into the borehole;
d) the fluid should be able to form an impervious filter cake, or deposit, on the bore side of the hole, in order to prevent undue loss of fluid into the earth formation.
Broadly, drilling fluids may be oil-based or water-based liquid systems.
For a particular use, the choice of one kind of drilling fluid involves a careful balance among the advantages and disadvantages of each one of them, where the conditions of the well to be drilled as well as the characteristics of the oil and gas field to be perforated are of paramount importance.
Among the advantages it can be said that oil-based fluids promote: a) better well stability, especially in the case of clayish formations; b) formation of a thinner filter cake than that observed in the case of water-based fluids; c) more effective penetration in salt formations without causing washout or enlargement of the borehole and other benefits well-known to those skilled in the drilling art.
The most beneficial property of the said oil-based fluids is their ability to lubricate the drill string and the bottom well tools. Based on such property it is possible to drill highly inclined wells, as is typically done in deep water offshore drilling, or horizontal wells drilling.
In those highly inclined wells, the drilling tubes are placed on the well bottom and therefore the contact with the rock is very much increased, requiring a higher force to rotate the drill string and, consequently, exerting more torque over the drill string structure.
Under these circumstances, the best way to preserve the mechanical strength of the drill string and avoid its sticking to the rock is to make use of oil-based fluids, since they provide a thin layer of lubricant between the rock and the drill string, making those contacts softer.
In spite of that many advantages of the oil-based fluids, they bear some disadvantages.
In general, oil-based drilling fluid operations are highly costly and more difficult to proceed than those employing water-based drilling fluids.
Depending upon the hole depth, such costs can become significant. Nevertheless, the use of those fluids is preferred based on statistic data indicating that they can prevent the occurrence of the well washout and enlargement, those factors leading to longer drilling operation duration and, consequently, rendering them more expensive.
The disposal of the oily cutting rocks produced in such type of operation is another problem, mainly when it is done in offshore drilling. In this case, the said oily cutting rocks must be washed with non-polluting detergents or transported onshore before being suitably disposed off or otherwise recovered.
Increasingly stringent regulations on the adverse environmental effects of the drilling fluids require the development of non-polluting substitutes.
Oil-based drilling fluids are inverted emulsions made up from a three-phase system comprising, basically, an oil, water and thinly divided solids which are complemented by specific emulsifiers, or emulsifier systems, alkaline compounds, weighting agents, fluid loss controlling agents and viscosity regulators. Such compounds are used to stabilize the overall system while providing the properties required for the adequate performance of the drilling fluid.
A huge number of oil-based drilling fluid compositions have been described in the literature.
When such oil-based drilling fluids are designed to be non-polluting, particularly, when they have to be used in offshore operations, the main concern is focused on the oil phase properties, because it is the main responsible component for the biodegradability properties of the overall composition.
Current compositions of biodegradable oil-based drilling fluids make increased use of esters prepared from the reaction of vegetable oil fatty acids with monoalcohols as the oil phase since the intrinsic non-polluting properties of the vegetable oils are preserved in the final product.
As an example, U.S. Pat. No. 4,631,136, U.S. Pat. No. 5,232,910 and U.S. Pat. No. 5,252,554 teach the use of selected fatty acid esters from vegetable oils in formulations of oil-based drilling fluids, but restricts, in the claims, the ester molecules to those bearing from two to twelve carbon atoms in their alcohol moiety.
U.S. Pat. No. 6,218,342 too teaches the use of the same kind of esters in the composition of an oil-based drilling fluid presented in Example 7.
However, the experimental evidence presented by those patents is based only on laboratory tests while the Applicant interest is directed to using such formulations in very deep water (above 1000 m water depth) drilling operations where the variation of the fluid temperature may reach up to 46° C., with the consequent strong viscosity increase that renders more difficult to bring the fluid up to the platforms and there separate the cutting rocks. Augmented viscosity further promotes a considerable increase in the pumping pressure.
Very deep water drilling requires therefore an improvement in oil-based drilling fluid compositions. Such compositions as well as the drilling process making use of same are described and claimed in the present invention.